Ethylene is a plant hormone which is a powerful regulator of plant metabolism, acting, and interacting with other plant hormones in trace amounts. Ethylene is a gas under normal physiological conditions. Even at low concentrations, ethylene has profound hormonal effects on plants.
The effects of ethylene, whether produced by the plant itself or applied exogenously, are numerous, dramatic, and of considerable commercial importance. Among the diverse physiological effects are the following: leaf abscission; fading in flowers; flower wilting; leaf yellowing; leaf epinasty; and stimulation of ripening in fruits and vegetables. Ethylene promotes senescence in plants, both in selected groups of cells and in whole organs, such as, fruits, leaves, or flowers. Senescence is the natural, genetically controlled degenerative process which usually leads to death in plants.
Normally, ethylene production from plant tissue is low. Large quantities of ethylene, however, are produced during ripening and senescence processes. A large amount of ethylene is also produced following trauma caused by chemicals, temperature extremes, water stress, ultraviolet light, insect damage, disease, or mechanical wounding. Ethylene produced by plants under such trauma conditions is referred to as "wound ethylene" or "stress ethylene". In fruits and vegetables, the stimulation of ethylene production by cuts or bruises may be very large and bear considerably on storage effectiveness. Ethylene-induced leaf browning is a common basis for loss in many plants, including lettuce and tobacco. In some tissues, exposure to only a small amount of ethylene may cause an avalanche of ethylene production in adjacent plants or plant tissues such as fresh produce. This autocatalytic effect can be very pronounced and lead to loss of fruit quality during transportation and storage.
Current technologies that specifically address post-harvest storage life have been in existence for decades and are hampered by such problems as high cost, side effects, and an inability to completely shut off ethylene production. Included in this group are controlled atmosphere (CA) storage, chemical treatment, packaging, and irradiation.
CA facilities slow ethylene biosynthesis through: (i) low temperature, (ii) reducing the oxygen level below 3%, and (iii) elevating the carbon dioxide level in the storage area to the 3%-5% range. Expensive scrubbers are sometimes added which reduce ethylene already respired to the atmosphere. Drawbacks are that CA facilities are expensive to construct, have a high utility cost, and are unable to completely eliminate ethylene production and side effects. Also, CA storage techniques can only control external ethylene and not that which resides inside the plant tissue. CA storage can also lead to undesirable side effects: injury can result from high CO.sub.2 levels, low O.sub.2 levels, or low temperature.
Another treatment is to limit the ethylene biosynthesis in the plant tissue through chemical treatment. Aminoethoxyinylglycine (AVG), an analog of the antibiotic rhizobitoxine, is one such inhibitor. However, AVG cannot be used as a chemical additive in foods due to its high toxicity. Silver thiosulfate (STS) is also effective in slowing fruit ripening and flower fading, but is also toxic and cannot be used on foods. Further, STS only works with certain flowers and often causes black spotting.
Recently, molecular genetic approaches leading to transgenic plants with impaired biosynthesis of ethylene have been reported. Hamilton, et al., identified a cDNA clone for tomato EFE (pTOM13) by inhibiting ethylene synthesis with an antisense gene expressed in transgenic plants. Oeller, et al., showed that expression of antisense RNA to the rate-limiting enzyme in the biosynthetic pathway of ethylene, 1-aminocyclopropane-1-carboxylate synthase, inhibits fruit ripening in tomato plants. Klee, et al., cloned the gene encoding ACC deaminase, from soil bacteria, and introduced it into tomato plants. Reduction in ethylene synthesis in transgenic plants did not cause any apparent vegetative phenotypic abnormalities. However, fruits from these plants exhibited significant delays in ripening, and the mature fruits remained firm for at least 6 weeks longer than the non-transgenic control fruit.